The present invention relates to a method for treatment of laundry in a household washing machine with a foam-forming washing liquor, comprising at least one washing process and at least one rinsing process.
The detergents used in conventional washing machines are based on surfactants. These active washing substances are able, by virtue of their amphiphile molecular structure, to form a thin layer on the surface of the water and thereby reduce the surface tension of the water. It is also the surfactants which lend the water its ability to form foam. The lowering of the surface tension of the aqueous phase by accumulation of surfactants at the boundary surface between water and air has the result that by the introduction of air, such as by a mechanical input for example on rotation of a washing drum, small air bubbles are created and can be partially stabilized by the embodiment of a surface layer without them coalescing too quickly. This means that a dispersion of air in the washing liquor forms the basis for the generation of foam. The foams involved in washing usually involve metastable foams which have a certain, but limited, lifetime. The lifetime is influenced by different variables, for example by the type of surfactant, by its concentration, by the intensity of the air input, by the diameter of the bubbles produced. If the lifetime of the foam is very short, this can be termed an unstable foam.
A distinction is also made between two types of foams: Spherical foams and polyhedral foams. With a foaming liquid such as a washing liquor for example both types generally occur, with the polyhedral foam to be found above the spherical foam.
Small-pore polyhedral foams in particular can however be a hindrance or even be damaging in a washing cycle in a household drum washing machine. These types of small-pore polyhedral foams can form a robust foam carpet which gets into the space between inner drum and tub, remains stuck there and is difficult to pump away. During the rinsing spin cycle this also prevents the spun-off water being transported away. This in its turn leads to an additional buildup of foam. Without foam removal measures this frequently leads to overfoaming of the device, i.e. the foam especially forcing its way out of the openings of the tub, such as the tub ventilation and the filler hose for example. Foam can also have a disadvantageous effect on the pump for pumping away the used washing liquor.
For this reason a number of methods have already been developed in the prior art in order to remove surplus foam. In the prior art this has primarily been attempted by using mechanical methods.
DE 43 34 969 A1 describes a device for destroying detergent foam in a washing machine with a rotatable drum and a tub which has a facility for creating hot air which is directed through an inlet in the tub wall between the tub and the drum.
According to the disclosure of DE 41 04 151 A1 unwanted foam occurring in a washing machine is destroyed by switching on a heating device.
In EP 0 278 239 A1 a disproportionate foam formation in the main wash cycle, which is to be observed during the heating of washing liquid in the tub, is removed by adding a limited amount of cold water and/or by temporarily switching off the tub heating.
The only chemical measure for countering foam known in the state of the art is to introduce fabric softeners at a relatively early stage of the rinsing process since, because of their composition from cationic surfactants, these have a defoaming effect.
DE 102 34 472 A1 specifies different measures for removal of foam, including automatic dispensing of a substance for inhibition of the foam.
Most foam removal measures known in the prior art are however associated with disadvantages, such as increased water consumption, increased power consumption from additional heating and longer idle times which increase the overall duration of a program.
As well as the undesired effects of detergent foam in household washing machines, it has also been shown however that foam can also have entirely advantageous effects. Thus foam attenuates the falling action of the laundry during washing, and this enables the mechanical input to be reduced despite a continuous rotational movement of the drum. This makes a more gentle wash for delicate fabrics possible, such as wool and silk for example. In conventional wool washing methods there is generally provision for long idle periods in which the drum is moved hardly at all in order to keep the mechanical effect on the laundry as low as possible.